The extra layer rule
The newborn is still learning to regulate its own temperature, and the most reliable reference is you: in the same environment, it needs one more layer than the adult is wearing comfortably. Above 24 degrees indoors, a bodysuit will do the trick. As the temperature drops, pants, a jumpsuit over the bodysuit and, for sleeping, a sleeping bag come into play.
The most confusing detail: the blanket on your lap and the sleeping bag are also layers. If the baby is wearing a bodysuit, pants and sleeping bag, he already has three layers. Be careful not to add a blanket over the set and overheat it.